FAULT-BOUNDED NEOGENE SEDIMENTARY DEPOSITS IN THE SANTA ROSA MOUNTAINS, SOUTHERN CALIFORNIA: CRUSTAL STRETCHING OR TRANSPRESSIONAL UPLIFT?

The Santa Rosa Mountains (SRM) on the W side of the Salton Trough in southern California display E-dipping low-angle faults that place brittly-deformed granitoids and associated Neogene sedimentary rocks against crystalline rocks of the SRM core (Sharp, 1979). Recent workers (Frost et al., 1996; Axen and Fletcher, 1998) interpret the low-angle faults as top-to-the-E normal-slip structures that played a role in extensional evolution of the San Andreas Fault zone and Salton Trough—a model that requires significant translation on the low-angle structures. Our mapping in the southern SRM constrains the amount of this translation. There, Neogene sedimentary rocks (Zosel sequence) are separated from the basement core of the SRM by the Zosel Fault (ZF), an E-dipping normal-slip structure that is steep (>65°) along much of the range front but shallow (~20°) near the range crest. This orientation seems to invoke a rolling-hinge geometry for the ZF—a geometry compatible with extensional exhumation of the SRM footwall and large horizontal top-to-the-E translation on the ZF. However, this interpretation is complicated by provenance data for clasts in the Zosel sequence. Most of the clasts are unlike basement rocks in the SRM, and were sourced from terrains W of the range (Cox et al, this volume). The Zosel clasts are not all exotic, however: they include tectonites found only in the basement core of the SRM. This poses a paradox: how can the hanging-wall Zosel sequence contain local SRM basement clasts throughout, yet still have been juxtaposed against the SRM from some initial westward position by significant top-to-the-E translation of the hanging wall? We speculate that the Zosel sequence formed largely in place relative to the SRM core, with minimal horizontal disruption on the ZF. The sequence was sourced from a granitoid uplift to the SW (Cox et al, this volume). Tonalitic Zosel exotics are akin to tonalite exposed within the San Jacinto Fault zone (SJF) near White Wash (WW), well to the NW of the SRM. Restoration of ~20 km of right slip on the Clark Fault strand of the SJF places the WW outcrops close to similar tonalite near the Rockhouse Canyon area (Sharp, 1967), where they could have sourced tonalitic sediment in the Zosel sequence. Pleistocene transpressional tectonics in the SRM region disrupted this paleogeographic setting.